My truck (and her car for that matter) runs good on mag, but I always wanted to try the light bulb test. So tonight, I did. 1156 bulb and analog volt meter. At idle, I got 6.5 volts and a brighter than I expected light bulb. As the rpm increased, bulb got brighter. At between 25 and 30 volts, the bulb burned out. I think I can safely assume my mag is in prett good shape.
Can you tell exactly how you did this please as I don't want to assume.
Hook up lightbulb - rev it up - burn it out.
I bought a bulb socket from Autozone. It was actually one of the hemispherical tag light deals for older Ford trucks. The bulb that came in it is not an 1156, but has the same base. So I tossed the bulb and the globe and soldered a wire to the base. Put an 1156 bulb in the base. Aligator clips would be nice, but I didn't have any, so I stripped the wires back 3/4" and wrapped them around the leads to my analog multimeter. One lead to ground. The other lead to the mag post. (I actually checked it at the coil box terminal so I didn't have to remove the floor boards). I had my son run the throttle for me.
This was on my truck. I want to do Karen's Touring, too. I was just running out of daylight last night.
Here are the complete details on the magneto output test developed by John Regan several years ago.
Since then John is reproducing the precision St Louis magneto tester which is less complicated to use for the non electrically inclined. St Louis Electric Works Magneto Tester If you do much work on Model T's this is an excellent foolproof magneto test tool.
If you don't plan to use and 1156 bulb (see complete details why below), analog meter or place the bulb in parallel (if you need a diagram for this contact me.)with the meter leads don't bother as the results will not be conclusive.
Go to your local auto parts store and buy a #1156 bulb. This is commonly used as a back up light bulb in modern cars. This bulb will come close to simulating the load of a typical Model T coil. You may want to pick up a socket for it too and put some wires on it to make a regular test light out of it.
Connect the bulb across the magneto output and ground while you running the car on the battery with the emergency brake pulled all the way back and set. Using an analog voltmeter check the AC voltage across this bulb as a load.
Provide the following test results back to me via email:
AC Voltage reading at engine idle:
Lamp Brightness at engine idle:
AC Voltage reading at engine moderate speed:
Lamp Brightness at engine moderate speed:
AC Voltage reading at engine high speed:
Lamp Brightness at engine high speed:
A good magneto will produce at least 7–8 volts AC across this load at a brisk idle.
Post the results here and we will let you know the relative health of your magneto. If your magneto output passes this test it has sufficient output to power properly adjusted coils.
There are several reasons to use the 1156 bulb. You are correct it is a 12V bulb (12.8V rated actually) and is rated at 2.1A when connected to 12.8V The reason it is a good choice for testing the magneto is that the 1156 bulb draws just about 1.3 Amps at 8V and Model T magneto at a brisk idle should put out about 6-8V. Since 1.3 Amps is the average load of 1 properly adjusted coil, testing the magneto output voltage at idle to see if it will power this bulb to about 8V will help one decide that the magneto is OK and the coils are the problem or that the coils might be OK and the magneto is the problem. The bulb test is NOT an attempt to do a complete operational check of the magneto since that is really unnecessary. The bulb is the stable "load" to use for the test since at this point in the test both the magneto and the coils are suspect. If the magneto will put out enough voltage (8v) at idle and get even brighter as you rev the engine, you have proven the magneto good and can find out what is wrong with the coils. Further testing of the magneto at full RPM with 28V bulb is probably not really helpful to resolve the simple question of why the car won't run on magneto. The 1156 bulb is available almost everywhere for pocket change and typical 28V bulbs will NOT draw the 1.3 amps at T idle speed. Thus using a different bulb just makes things more complicated since if the magneto will power the 28V bulb to 8V at idle, it doesn't mean anything because at 8V the 28V bulb is likely drawing WAY LESS than 1.3 amps. Unless you load the magneto to 1.3 amps there is no way to be sure it will be able to reach 8V with a coil. By using the 1156, even if you burn the bulb out by over revving, you have proved the magneto is good. For about $1 you have solved the question about which is bad - magneto or coils and that is a cheap test. Complicated tests are not as good as simple ones that are easy to repeat and reliable. On tour you won't find some exotic industrial bulb number at your local auto parts store but around here 1156 is sold at the drug store.
Ron the Coilman
You have to be very careful about where you connect the meter/bulb for this test. The last thing you want to do is make a mistake and get battery voltage connected to the hogshead terminal as it can demagnetize the magnets.
It is safest to execute the test with the engine running on battery and make the test connections at the hogshead terminal/ground.
Ron the Coilman
Can you give me an opinion on the KRWilson Magneto Tester. I got one that I use a lot and would like to know how reliable or how accurate they are.
I shall defer to Herr Doktor Professor Regan.
He conducted the comparative analysis of many magneto testers we collected before settling on the reproducing the St Louis Electric magneto tester.
He will give you a short???? answer.
Ron the Coilman
Whats "a brisk idle" or "full rpm" anyway - number please!
Not trying to be flippant but since T's don't have a tachometer - things like "Brisk Idle" and "full RPM" are only meaningful enough to hopefully let one decide that the mag is good or not. There are those who with effort can get their T to idle very very slow indeed and its can even be impressive but that speed is not typical and is not obviously a "brisk idle". I totally agree that the terms used are not scientific but I coined the words "brisk idle" so will take the blame and you are correct it is NOT scientific but neither is the entire test other than it seems to help folks make the decision as to the problem being coils or mag and that is all it is intended to do - nothing more. Most of the time such a test produces "obvious" results. If one cannot decide based on the test then the problem falls into a rather small catagory that will indeed need more testing with better equipment. If you have an analog meter then the test costs about a buck and you get way more bang for that buck than for most things you buy at that price.
There are a ton of "magneto testers" from the T era and the vast majority of them are simply AC volt meters with words like "Strong, Medium, Weak...etc" being printed along the top. The instruction booklets that I have been able to read always tell you to compare the readings you are getting with YOUR magneto to readings then take with the same instrument on a GOOD magneto. Not one of them defines what GOOD is but it strikes me that if you have a car nearby that has a GOOD magneto for sure then why not pop the 4 coils in question into that car and see if it runs ha ha.
I don't have specific knowledge of the tester you named but would be happy to evaluate it and tell you exactly what it is testing and how it is designed to work. The St. Louis Tester is by far the best thing I found among the testers. I am not saying that because I make them because I take no credit for the engineering of the thing. I simply reproduce it. It tests the magneto and gives you an actual GO or NO GO type test in that if the magneto in question cannot make the needle rise above the 1914 Mark on the scale - the magneto is not going to run coils reliably. No comparison to any other magneto is involved. It is an amazingly accurate tool and I would invite anyone who has not used it to check it out. I have 3 originals that I found and purchased and one of them is a different brand but also made in St. Louis and I am not sure if it is a spinoff or copy or perhaps just a rebranding of the same tester since internally it is identical to the St. Louis Tester.
Hope this helps.
Ok, Ron, not meaning to be completely thick, but when you say "Using an analog voltmeter, check the AC voltage across this bulb as a load" does that mean I put the meter in series or parallel with the bulb?
Electrically challenged yours,
This is from an old Vintage Ford magazine
Dale's post above has the correct procedure. Be sure to use an ANALOG meter.
Ron the Coilman
Ok, thanks, Dale and Ron.
Ok, one last stupid question (at least for today):
I have a multi-meter, analog, to measure the voltage, but it does have a 9 volt battery inside to operate it. If used as in the diagram above, is there any danger of damaging anything?
Others may have more information, but I'm thinking the battery is for testing resistance only. I bet it will check voltage without the battery even in there.
Hal is correct. The battery in these analog test meters is for measuring resistance. Not so of the digital type meters. There the battery is to power the meter digital circuitry too and thus needed for any AC or DC readings too. No matter since using a digital type meter will yield false results anyway. You can us a digital type meter for measuring things on a T but not when the engine is running. Since most of the meaningful measurements you need to take are when the engine is running, you really need an analog meter to test electrical things on your T. A cheap one can be bought for under $10 at most hardware or home improvement big box stores in their electrical departments.
I spin 'em up on a stand to prevent embarrassment and lost wages after assembly - the one I spun up Thursday had 12 volts at 500 rpm. It's got a very odd looking field coil on it and a set of magnets recharged by moi.
If that was taken with an accurate analog AC voltmeter with an 1156 as a load then that is very good since that is probably slower than a "brisk idle" ha ha.
Ok, did some playing with the engine currently on the stand here. This was not scientifically done, nor is this engine assumed identical to all others as it has a oddball field coil on it. But this is what I got using an "unloaded" conventional modern a/c volt meter:
At 35 RPM -yup you read right - 35. I got 0.6 volts
At 98 RPM - I got 1.6 volts
At 120 RPM - I got 2 volts.
At 500 RPM I got 12 volts on the internally loaded Model T Magneto Meter found on a forum members website (can't remember where I bought it right now - will the vendor speak up?)
I used the regular meter for the low numbers because the Model T Magometer (whatever) only flinched at the lower speeds and I wouldn't tell if it was .5 volts or 2.5 volts. Maybe next time I'll take a more serious approach and load it with a light bulb (or buzz coil) before taking a reading.
Tim, You are still alive!!!!!
funny Rob. Yup, still alive. Up to my armpits in Model T work and don't really have time to play on here.
Wait Rob, I can do better...
How's she bootin'er? I'm alive Eh, how about next time I'm up there we meet at Timmy Ho's for a Double-double and a Timbits, I'll either have to bring my runners or borrow a loonie. I'll pay you back though, maybe bring a two-four or a texas mickey over later, watch some hockey eh?
Ok, this hoser is out of material. Well I'm not but your "_ the dog all weekend" might be taken wrong by 99% of these readers! Back to T talk - when are you going to bring that engine down?